Minimizing Lock and Unlock Calls in Protected Mode

ID: Q74197

The information in this article applies to:

Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

SUMMARY

When Microsoft Windows is running in protected mode (standard mode or enhanced mode), the selector of a movable memory block does not change even though Windows may move the block in physical (standard mode, enhanced mode) or linear (enhanced mode) memory.

Applications that run only in protected mode can take advantage of this behavior to minimize the number of GlobalLock() and GlobalUnlock() calls.

MORE INFORMATION

The following code demonstrates how to allocate a global memory object:


   HANDLE hMem;
   LPSTR lpstr;

   if ((hMem = GlobalAlloc(GMEM_MOVEABLE, cb)) != NULL)
      if ((lpstr = GlobalLock(hMem)) != (LPSTR)NULL)
         {
         // use the memory
         }

The following code demonstrates how to free the global memory object allocated above:


   if (GlobalUnlock(hMem) != (HANDLE)NULL)
      GlobalFree(hMem);

Leaving movable global memory objects locked does not impose a memory consumption penalty in protected mode.

This technique is not appropriate for an application that runs in real mode under versions of Windows prior to version 3.1. In real mode, Windows manages a limited amount of memory (less than 640K) by moving and discarding memory blocks. When an application locks a memory block, Windows cannot move or discard the block. Therefore, an application that runs in real mode must lock an object only while the object is in use.

Additional query words: 3.00 3.10

Keywords : kb16bitonly
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :